This was discussed quite a long time ago, and someone (sorry, I forget who) with the wiring schematics reckoned it was possible. Though I'm not so sure.
I believe the voice synth is driven by the instrument cluster, and as part of this update, the Sagem cluster was replaced with a Magnetti Marelli one. The obvious differences are that the new dash shows "
OK" on start-up, you can get the tyre pressures to show when you're driving, and the trip reset is via the wiper stalk rather than a button sticking out of the dial.
I'm guessing that it's more than coincidence that the voice synth went at the same time. I reckon they dropped the functionality when speccing the new cluster.
As for the system going to 'speech', this suggests that something is manipulating the muting wire to the stereo. If you have a wired hands-free, then that will almost certainly be the culprit. If not, then you've either got a wiring fault somewhere (damaged loom) or the stereo's gone a bit flakey.